well the 512 MB version of the GTX isn't just an increase of RAM, but it is also clocked at or around 550MHz core and 1800MHz memory (the origional is at 430MHz and 1200MHz). But I haven't seen any benchmarks from it and what the price is marked at(although Nvidia made this to compete the X1800XT so it might be priced around that) so I would wait.
But from what I hear that because the new 512 version w/ 24 pipes that the GTX has and with those speeds it will probably will out perform it. I wonder how far you can OC those things.
But the X1800XT can run some games such as FarCry that have HDR support w/ fulll AA which the GTX can't be done (I think all it can do with HDR enabled in FarCry is 2x AA), except HL2: Loast Coast which the GTX can fun HDR w/ full AA.
I plan on getting the new GTX w/ 512MB but I will wait untill I get some benchmarks.